comile sass to css with autoprefixer and webpack

package.json

{
  "name": "sass-autoprefixer-sample",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "build-css": "rm -rf public && webpack --debug --config webpack.config.js"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "autoprefixer": "^7.1.5",
    "css-loader": "^0.28.7",
    "extract-text-webpack-plugin": "^3.0.1",
    "node-sass": "^4.5.3",
    "postcss-loader": "^2.0.8",
    "sass-loader": "^6.0.6",
    "style-loader": "^0.19.0",
    "webpack": "^3.8.1"
  }
}

sample.css

.sample {
  color: #00008B;
  font-size: large;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  background-image: -webkit-linear-gradient(left, red, orange, yellow, green, blue, indigo, violet);
  background-image: linear-gradient(left, red, orange, yellow, green, blue, indigo, violet);
  -webkit-user-select: none;
     -moz-user-select: none;
      -ms-user-select: none;
          user-select: none; }

sample.sass

@charset "UTF-8"

$darkblue: #00008B

.sample
  color: $darkblue
  font-size: large
  display: flex
  background-image: linear-gradient(left, red, orange, yellow, green, blue, indigo, violet)
  user-select: none

webpack.config.js

"use strict"

const path = require("path");
const webpack = require("webpack");
const ExtractTextPlugin = require("extract-text-webpack-plugin");
const autoprefixer = require("autoprefixer");

module.exports = {
  entry: {
    sample: path.join(__dirname, "src/css/sample.sass"),
  },
  output: {
    path: path.join(__dirname, "public/css"),
    filename: "[name].css"
  },
  module: {
    rules: [
      {
        test: /\.s[ac]ss$/,
        use: ExtractTextPlugin.extract({
          fallback: "style-loader",
          use: [
            "css-loader",
            {loader: 'postcss-loader', options: { plugins: [autoprefixer({browsers: [">= 0.1%"]})]}},
            "sass-loader"
          ]
        })
      }
    ]
  },
  plugins: [
    new ExtractTextPlugin({ filename: '[name].css', disable: false, allChunks: true })
  ]
};